Slime Monsters

Slime Monsters are a set of monsters, identified as slightly transparent blobs that slowly hop toward the player in an attempt to attack. They are most likely going to be the first enemies you find in the game. They are the most common enemies in Terraria, able to spawn at any time of day and in nearly every biome. There are many types, all differing in appearance, stats, and behavior: As a general rule, however, the stronger the slime, the higher and more frequently they will jump at the player. They will all drop gel and coins.

As of 1.3, slimes can sometimes spawn with items inside them. This has been observed with ores, potions, and coins, and the items are visible inside them. They will always drop a number of them along with their normal drops.

Slimes have a particular jump pattern. It first jumps 2 small jumps (about 6 blocks) then jumps a higher jump. (about 10 blocks)

Because it can only jump to move, it may get stuck in a small space.

Yellow Slimes and Purple Slimes seem to spawn nearer to the edge of the world.

A player can tell when a slime is about to jump by watching the speed of its animation. When its animation is at full speed the slime will then jump.

Green Slimes usually are only found in the forest biomes.

The Royal Gel, an item obtained from slaying King Slime, makes all slimes docile towards the player with the exception of Hoppin' Jacks, Spiked Slimes, and King Slime himself. Please note Gastropods are considered slimes by this rule and are also rendered docile even though killing them does not count for the achievement "Gelatin World Tour".

Slimes become hostile in behavior at night and in dark areas such as in caverns.

Trivia

Virtually identical slime-like enemies of various colors (as in Terraria) star prominently in the 1984 arcade title "The Tower of Druaga" and it's respective 2008 anime series of the same name.